SunlightSunlight is a portion of the electromagnetic radiation given off by
the Sun, in particular infrared, visible, and ultraviolet light. On
Earth, sunlight is filtered through Earth's atmosphere, and is obvious
as daylight when the
SunSun is above the horizon. When the direct solar
radiation is not blocked by clouds, it is experienced as sunshine, a
combination of bright light and radiant heat. When it is blocked by
clouds or reflects off other objects, it is experienced as diffused
light. The
World Meteorological OrganizationWorld Meteorological Organization uses the term "sunshine
duration" to mean the cumulative time during which an area receives
direct irradiance from the
SunSun of at least 120 watts per square
meter.[1] Other sources indicate an "Average over the entire earth" of
"164 Watts per square meter over a 24 hour day".[2]
The ultraviolet radiation in sunlight has both positive and negative
health effects, as it is both a principal source of vitamin D3 and a
mutagen.
SunlightSunlight takes about 8.3 minutes to reach
EarthEarth from the surface
of the Sun. A photon starting at the center of the
SunSun and changing
direction every time it encounters a charged particle would take
between 10,000 and 170,000 years to get to the surface.[3]
SunlightSunlight is a key factor in photosynthesis, the process used by plants
and other autotrophic organisms to convert light energy, normally from
the Sun, into chemical energy that can be used to fuel the organisms'
activities.

Measurement[edit]
Researchers can measure the intensity of sunlight using a sunshine
recorder, pyranometer, or pyrheliometer. To calculate the amount of
sunlight reaching the ground, both the eccentricity of Earth's
elliptic orbit and the attenuation by
Earth's atmosphereEarth's atmosphere have to be
taken into account. The extraterrestrial solar illuminance (Eext),
corrected for the elliptic orbit by using the day number of the year
(dn), is given to a good approximation by[4]

where dn=1 on January 1st; dn=32 on February 1st; dn=59 on
March 1 (except on leap years, where dn=60), etc. In this formula
dn–3 is used, because in modern times Earth's perihelion, the
closest approach to the
SunSun and, therefore, the maximum Eext occurs
around January 3 each year. The value of 0.033412 is determined
knowing that the ratio between the perihelion (0.98328989 AU)
squared and the aphelion (1.01671033 AU) squared should be
approximately 0.935338.
The solar illuminance constant (Esc), is equal to 128×103 lx.
The direct normal illuminance (Edn), corrected for the attenuating
effects of the atmosphere is given by:

E

d
n

=

E

e
x
t

e

−
c
m

,

displaystyle E_ rm dn =E_ rm ext ,e^ -cm ,

where c is the atmospheric extinction and m is the relative optical
airmass. The atmospheric extinction brings the number of lux down to
around 100 000.
The total amount of energy received at ground level from the
SunSun at
the zenith depends on the distance to the
SunSun and thus on the time of
year. It is about 3.3% higher than average in January and 3.3% lower
in July (see below). If the extraterrestrial solar radiation is 1367
watts per square meter (the value when the Earth–
SunSun distance is 1
astronomical unit), then the direct sunlight at Earth's surface when
the
SunSun is at the zenith is about 1050 W/m2, but the total amount
(direct and indirect from the atmosphere) hitting the ground is around
1120 W/m2.[5] In terms of energy, sunlight at Earth's surface is
around 52 to 55 percent infrared (above 700 nm), 42 to 43 percent
visible (400 to 700 nm), and 3 to 5 percent ultraviolet (below
400 nm).[6] At the top of the atmosphere, sunlight is about 30%
more intense, having about 8% ultraviolet (UV),[7] with most of the
extra UV consisting of biologically damaging short-wave
ultraviolet.[8]
Direct sunlight has a luminous efficacy of about 93 lumens per
watt of radiant flux. This is higher than the efficacy (of source) of
most artificial lighting (including fluorescent), which means using
sunlight for illumination heats up a room less than using most forms
of artificial lighting.
Multiplying the figure of 1050 watts per square metre by 93 lumens per
watt indicates that bright sunlight provides an illuminance of
approximately 98 000 lux (lumens per square meter) on a perpendicular
surface at sea level. The illumination of a horizontal surface will be
considerably less than this if the
SunSun is not very high in the sky.
Averaged over a day, the highest amount of sunlight on a horizontal
surface occurs in January at the
South PoleSouth Pole (see insolation).
Dividing the irradiance of 1050 W/m2 by the size of the sun's
disk in steradians gives an average radiance of 15.4 MW per
square metre per steradian. (However, the radiance at the centre of
the sun's disk is somewhat higher than the average over the whole disk
due to limb darkening.) Multiplying this by π gives an upper limit to
the irradiance which can be focused on a surface using mirrors:
48.5 MW/m2.
Composition and power[edit]

Solar irradiance spectrum above atmosphere and at surface. Extreme UV
and X-rays are produced (at left of wavelength range shown) but
comprise very small amounts of the Sun's total output power.

See also: Ultraviolet, Infrared, and Light
The spectrum of the Sun's solar radiation is close to that of a black
body[9][10] with a temperature of about 5,800 K.[11] The Sun
emits EM radiation across most of the electromagnetic spectrum.
Although the
SunSun produces gamma rays as a result of the nuclear-fusion
process, internal absorption and thermalization convert these
super-high-energy photons to lower-energy photons before they reach
the Sun's surface and are emitted out into space. As a result, the Sun
does not emit gamma rays from this process, but it does emit gamma
rays from solar flares.[12] The
SunSun also emits X-rays, ultraviolet,
visible light, infrared, and even radio waves;[13] the only direct
signature of the nuclear process is the emission of neutrinos.
Although the solar corona is a source of extreme ultraviolet and X-ray
radiation, these rays make up only a very small amount of the power
output of the
SunSun (see spectrum at right). The spectrum of nearly all
solar electromagnetic radiation striking the
Earth's atmosphereEarth's atmosphere spans
a range of 100 nm to about 1 mm
(1,000,000 nm).[citation needed] This band of significant
radiation power can be divided into five regions in increasing order
of wavelengths:[14]

Ultraviolet C or (UVC) range, which spans a range of 100 to
280 nm. The term ultraviolet refers to the fact that the
radiation is at higher frequency than violet light (and, hence, also
invisible to the human eye). Due to absorption by the atmosphere very
little reaches Earth's surface. This spectrum of radiation has
germicidal properties, as used in germicidal lamps.
Ultraviolet B or (UVB) range spans 280 to 315 nm. It is also
greatly absorbed by the Earth's atmosphere, and along with UVC causes
the photochemical reaction leading to the production of the ozone
layer. It directly damages
DNADNA and causes sunburn, but is also
required for vitamin D synthesis in the skin and fur of mammals.[15]
Ultraviolet A or (UVA) spans 315 to 400 nm. This band was
once[when?] held to be less damaging to DNA, and hence is used in
cosmetic artificial sun tanning (tanning booths and tanning beds) and
PUVA therapy for psoriasis. However, UVA is now known to cause
significant damage to
DNADNA via indirect routes (formation of free
radicals and reactive oxygen species), and can cause cancer.[16]
Visible range or light spans 380 to 780 nm. As the name suggests,
this range is visible to the naked eye. It is also the strongest
output range of the Sun's total irradiance spectrum.
InfraredInfrared range that spans 700 nm to 1,000,000 nm
(1 mm). It comprises an important part of the electromagnetic
radiation that reaches Earth. Scientists divide the infrared range
into three types on the basis of wavelength:

Published tables[edit]
Tables of direct solar radiation on various slopes from 0 to 60
degrees north latitude, in calories per square centimetre, issued in
1972 and published by Pacific Northwest
ForestForest and Range Experiment
Station,
ForestForest Service, U.S. Department of Agriculture, Portland,
Oregon, USA, appear on the web.[17]
Solar constant[edit]
Main article: Solar constant

Solar irradiance spectrum at top of atmosphere, on a linear scale and
plotted against wavenumber

The solar constant, a measure of flux density, is the amount of
incoming solar electromagnetic radiation per unit area that would be
incident on a plane perpendicular to the rays, at a distance of one
astronomical unit (AU) (roughly the mean distance from the
SunSun to
Earth). The "solar constant" includes all types of solar radiation,
not just the visible light. Its average value was thought to be
approximately 1366 W/m²,[18] varying slightly with solar
activity, but recent recalibrations of the relevant satellite
observations indicate a value closer to 1361 W/m² is more
realistic.[19]
Total solar irradiance (TSI) and spectral solar irradiance (SSI) upon
Earth[edit]
Total solar irradiance (TSI) – the amount of solar radiation
received at the top of
Earth's atmosphereEarth's atmosphere – has been measured since
1978 by a series of overlapping NASA and ESA satellite experiments to
be 1.361 kilo⁠watts per square meter (kW/m²).[18][20][21][22] TSI
observations are continuing today with the ACRIMSAT/ACRIM3, SOHO/VIRGO
and SORCE/TIM satellite experiments.[23] Variation of TSI has been
discovered on many timescales including the solar magnetic cycle [24]
and many shorter periodic cycles.[25] TSI provides the energy that
drives Earth's climate, so continuation of the TSI time series
database is critical to understanding the role of solar variability in
climate change.
Spectral solar irradiance (SSI) – the spectral distribution of the
TSI – has been monitored since 2003 by the
SORCESORCE Spectral Irradiance
Monitor (SIM). It has been found that SSI at UV (ultraviolet)
wavelength corresponds in a less clear, and probably more complicated
fashion, with Earth's climate responses than earlier assumed, fueling
broad avenues of new research in “the connection of the
SunSun and
stratosphere, troposphere, biosphere, ocean, and Earth’s
climate”.[26]
Intensity in the Solar System[edit]

Different bodies of the
Solar SystemSolar System receive light of an intensity
inversely proportional to the square of their distance from Sun. A
rough table comparing the amount of solar radiation received by each
planet in the
Solar SystemSolar System follows (from data in [1]):

The actual brightness of sunlight that would be observed at the
surface depends also on the presence and composition of an atmosphere.
For example, Venus's thick atmosphere reflects more than 60% of the
solar light it receives. The actual illumination of the surface is
about 14,000 lux, comparable to that on
EarthEarth "in the daytime
with overcast clouds".[27]
SunlightSunlight on
MarsMars would be more or less like daylight on
EarthEarth during a
slightly overcast day, and, as can be seen in the pictures taken by
the rovers, there is enough diffuse sky radiation that shadows would
not seem particularly dark. Thus, it would give perceptions and "feel"
very much like
EarthEarth daylight. The spectrum on the surface is slightly
redder than that on Earth, due to scattering by reddish dust in the
Martian atmosphere.
For comparison, sunlight on
SaturnSaturn is slightly brighter than Earth
sunlight at the average sunset or sunrise (see daylight for comparison
table). Even on Pluto, the sunlight would still be bright enough to
almost match the average living room. To see sunlight as dim as full
moonlight on Earth, a distance of about 500 AU
(~69 light-hours) is needed; there are only a handful of objects
in the
Solar SystemSolar System known to orbit farther than such a distance, among
them
90377 Sedna90377 Sedna and (87269) 2000 OO67.
Surface illumination[edit]
The spectrum of surface illumination depends upon solar elevation due
to atmospheric effects, with the blue spectral component dominating
during twilight before and after sunrise and sunset, respectively, and
red dominating during sunrise and sunset. These effects are apparent
in natural light photography where the principal source of
illumination is sunlight as mediated by the atmosphere.
While the color of the sky is usually determined by Rayleigh
scattering, an exception occurs at sunset and twilight. "Preferential
absorption of sunlight by ozone over long horizon paths gives the
zenith sky its blueness when the sun is near the horizon".[28]
See diffuse sky radiation for more details.
Spectral composition of sunlight at Earth's surface[edit]
The Sun's electromagnetic radiation which is received at the Earth's
surface is predominantly light that falls within the range of
wavelengths to which the visual systems of the animals that inhabit
Earth's surface are sensitive. The
SunSun may therefore be said to
illuminate, which is a measure of the light within a specific
sensitivity range. Many animals (including humans) have a sensitivity
range of approximately 400–700 nm,[29] and given optimal
conditions the absorption and scattering by Earth's atmosphere
produces illumination that approximates an equal-energy illuminant for
most of this range.[30] The useful range for color vision in humans,
for example, is approximately 450–650 nm. Aside from effects
that arise at sunset and sunrise, the spectral composition changes
primarily in respect to how directly sunlight is able to illuminate.
When illumination is indirect,
Rayleigh scatteringRayleigh scattering in the upper
atmosphere will lead blue wavelengths to dominate.
WaterWater vapour in the
lower atmosphere produces further scattering and ozone, dust and water
particles will also absorb selective wavelengths.[31][32]

Spectrum of the visible wavelengths at approximately sea level;
illumination by direct sunlight compared with direct sunlight
scattered by cloud cover and with indirect sunlight by varying degrees
of cloud cover. The yellow line shows the spectrum of direct
illumination under optimal conditions. The other illumination
conditions are scaled to show their relation to direct illumination.
The units of spectral power are simply raw sensor values (with a
linear response at specific wavelengths).

Variations in solar irradiance[edit]
SeasonalSeasonal and orbital variation[edit]
Further information:
InsolationInsolation and Sunshine duration
On Earth, the solar radiation varies with the angle of the sun above
the horizon, with longer sunlight duration at high latitudes during
summer, varying to no sunlight at all in winter near the pertinent
pole. When the direct radiation is not blocked by clouds, it is
experienced as sunshine. The warming of the ground (and other objects)
depends on the absorption of the electromagnetic radiation in the form
of heat.
The amount of radiation intercepted by a planetary body varies
inversely with the square of the distance between the star and the
planet. Earth's orbit and obliquity change with time (over thousands
of years), sometimes forming a nearly perfect circle, and at other
times stretching out to an orbital eccentricity of 5% (currently
1.67%). As the orbital eccentricity changes, the average distance from
the sun (the semimajor axis does not significantly vary, and so the
total insolation over a year remains almost constant due to Kepler's
second law,

2
A

r

2

d
t
=
d
θ
,

displaystyle tfrac 2A r^ 2 dt=dtheta ,

where

A

displaystyle A

is the "areal velocity" invariant. That is, the integration over the
orbital period (also invariant) is a constant.

If we assume the solar radiation power P as a constant over time
and the solar irradiation given by the inverse-square law, we obtain
also the average insolation as a constant.
But the seasonal and latitudinal distribution and intensity of solar
radiation received at Earth's surface does vary.[33] The effect of sun
angle on climate results in the change in solar energy in summer and
winter. For example, at latitudes of 65 degrees, this can vary by
more than 25% as a result of Earth's orbital variation. Because
changes in winter and summer tend to offset, the change in the annual
average insolation at any given location is near zero, but the
redistribution of energy between summer and winter does strongly
affect the intensity of seasonal cycles. Such changes associated with
the redistribution of solar energy are considered a likely cause for
the coming and going of recent ice ages (see: Milankovitch cycles).
Solar intensity variation[edit]
Further information: Solar variation
Space-based observations of solar irradiance started in 1978. These
measurements show that the solar constant is not constant. It varies
on many time scales, including the 11-year sunspot solar cycle.[24]
When going further back in time, one has to rely on irradiance
reconstructions, using sunspots for the past 400 years or
cosmogenic radionuclides for going back 10,000 years. Such
reconstructions have been done.[34][35][36][37] These studies show
that in addition to the solar irradiance variation with the solar
cycle (the (Schwabe) cycle), the solar activitiy varies with longer
cycles, such as the proposed 88 year (Gleisberg cycle), 208 year
(DeVries cycle) and 1,000 year (Eddy cycle).
LifeLife on Earth[edit]
The existence of nearly all life on
EarthEarth is fueled by light from the
Sun. Most autotrophs, such as plants, use the energy of sunlight,
combined with carbon dioxide and water, to produce simple sugars—a
process known as photosynthesis. These sugars are then used as
building-blocks and in other synthetic pathways that allow the
organism to grow.
Heterotrophs, such as animals, use light from the
SunSun indirectly by
consuming the products of autotrophs, either by consuming autotrophs,
by consuming their products, or by consuming other heterotrophs. The
sugars and other molecular components produced by the autotrophs are
then broken down, releasing stored solar energy, and giving the
heterotroph the energy required for survival. This process is known as
cellular respiration.
In prehistory, humans began to further extend this process by putting
plant and animal materials to other uses. They used animal skins for
warmth, for example, or wooden weapons to hunt. These skills allowed
humans to harvest more of the sunlight than was possible through
glycolysis alone, and human population began to grow.
During the Neolithic Revolution, the domestication of plants and
animals further increased human access to solar energy. Fields devoted
to crops were enriched by inedible plant matter, providing sugars and
nutrients for future harvests. Animals that had previously provided
humans with only meat and tools once they were killed were now used
for labour throughout their lives, fueled by grasses inedible to
humans.
The more recent discoveries of coal, petroleum and natural gas are
modern extensions of this trend. These fossil fuels are the remnants
of ancient plant and animal matter, formed using energy from sunlight
and then trapped within
EarthEarth for millions of years. Because the
stored energy in these fossil fuels has accumulated over many millions
of years, they have allowed modern humans to massively increase the
production and consumption of primary energy. As the amount of fossil
fuel is large but finite, this cannot continue indefinitely, and
various theories exist as to what will follow this stage of human
civilization (e.g., alternative fuels, Malthusian catastrophe, new
urbanism, peak oil).
Cultural aspects[edit]

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Claude Monet: Le déjeuner sur l'herbe

The effect of sunlight is relevant to painting, evidenced for instance
in works of
Claude MonetClaude Monet on outdoor scenes and landscapes.

Téli verőfény ("Winter Sunshine") by László Mednyánszky

Many people find direct sunlight to be too bright for comfort,
especially when reading from white paper upon which the sun is
directly shining. Indeed, looking directly at the sun can cause
long-term vision damage. To compensate for the brightness of sunlight,
many people wear sunglasses. Cars, many helmets and caps are equipped
with visors to block the sun from direct vision when the sun is at a
low angle. Sunshine is often blocked from entering buildings through
the use of walls, window blinds, awnings, shutters, curtains, or
nearby shade trees.
In colder countries, many people prefer sunnier days and often avoid
the shade. In hotter countries, the converse is true; during the
midday hours, many people prefer to stay inside to remain cool. If
they do go outside, they seek shade that may be provided by trees,
parasols, and so on.
In Hinduism, the sun is considered to be a god, as it is the source of
life and energy on earth.
Sunbathing[edit]
Main article:
SunSun tanning

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Sunbathing is a popular leisure activity in which a person sits or
lies in direct sunshine. People often sunbathe in comfortable places
where there is ample sunlight. Some common places for sunbathing
include beaches, open air swimming pools, parks, gardens, and sidewalk
cafes. Sunbathers typically wear limited amounts of clothing or some
simply go nude. For some, an alternative to sunbathing is the use of a
sunbed that generates ultraviolet light and can be used indoors
regardless of weather conditions. Tanning beds have been banned in a
number of states in the world.
For many people with light skin, one purpose for sunbathing is to
darken one's skin color (get a sun tan), as this is considered in some
cultures to be attractive, associated with outdoor activity,
vacations/holidays, and health. Some people prefer naked sunbathing so
that an "all-over" or "even" tan can be obtained, sometimes as part of
a specific lifestyle.
For people suffering from psoriasis, sunbathing is an effective way of
healing the symptoms.
Skin tanning is achieved by an increase in the dark pigment inside
skin cells called melanocytes, and is an automatic response mechanism
of the body to sufficient exposure to ultraviolet radiation from the
sun or from artificial sunlamps. Thus, the tan gradually disappears
with time, when one is no longer exposed to these sources.
Effects on human health[edit]
Main article: Health effects of sunlight exposure
The ultraviolet radiation in sunlight has both positive and negative
health effects, as it is both a principal source of vitamin D3 and a
mutagen.[38] A dietary supplement can supply vitamin D without this
mutagenic effect,[39] but bypasses natural mechanisms that would
prevent overdoses of vitamin D generated internally from sunlight.
Vitamin DVitamin D has a wide range of positive health effects, which include
strengthening bones[40] and possibly inhibiting the growth of some
cancers.[41][42]
SunSun exposure has also been associated with the timing
of melatonin synthesis, maintenance of normal circadian rhythms, and
reduced risk of seasonal affective disorder.[43]
Long-term sunlight exposure is known to be associated with the
development of skin cancer, skin aging, immune suppression, and eye
diseases such as cataracts and macular degeneration.[44] Short-term
overexposure is the cause of sunburn, snow blindness, and solar
retinopathy.
UV rays, and therefore sunlight and sunlamps, are the only listed
carcinogens that are known to have health benefits,[45] and a number
of public health organizations state that there needs to be a balance
between the risks of having too much sunlight or too little.[46] There
is a general consensus that sunburn should always be avoided.
Epidemiological data shows that people who have more exposure to the
sun have less high blood pressure and cardiovascular-related
mortality. While sunlight (and its UV rays) are a risk factor for skin
cancer, "sun avoidance may carry more of a cost than benefit for
over-all good health."[47] A study found that there is no evidence
that UV reduces lifespan in contrast to other risk factors like
smoking, alcohol and high blood pressure.[47]
Effect on plant genomes[edit]
Elevated solar UV-B doses increase the frequency of
DNADNA recombination
in
Arabidopsis thalianaArabidopsis thaliana and tobacco (Nicotiana tabacum) plants.[48]
These increases are accompanied by strong induction of an enzyme with
a key role in recombinational repair of
DNADNA damage. Thus the level of
terrestrial solar UV-B radiation likely affects genome stability in
plants.
See also[edit]

Solar radiationSolar radiation – Encyclopedia of Earth
Total Solar
Irradiance (TSI) Daily mean data at the website of the
National Geophysical Data Center
Construction of a Composite Total Solar
Irradiance (TSI) Time Series
from 1978 to present by World Radiation Center,
Physikalisch-Meteorologisches Observatorium Davos (pmod wrc)
A Comparison of Methods for Providing Solar Radiation Data to Crop
Models and Decision Support Systems, Rivington et al.
Evaluation of three model estimations of solar radiation at 24 UK
stations, Rivington et al.
High resolution spectrum of solar radiation from Observatoire de Paris
Measuring Solar Radiation : A lesson plan from the National
Science Digital Library.
Websurf astronomical information: Online tools for calculating Rising
and setting times of Sun, Moon or planet, Azimuth of Sun, Moon or
planet at rising and setting, Altitude and azimuth of Sun, Moon or
planet for a given date or range of dates, and more.
An Excel workbook with a solar position and solar radiation
time-series calculator; by Greg Pelletier
ASTM Standard for solar spectrum at ground level in the US (latitude
~37 degrees).
Detailed spectrum of the sun at Astronomy Picture of the Day.